I'm sure my statistics/math/politics is lacking something, but why isn't a third-party vote instead a -1 to both primary candidates? Is it naive to consider the vote of each registered voter as a potential gain for each candidate that is removed in the case for each name not chosen? We might as well say that everyone who voted outside the bipartisan menu as good as skipped the polls entirely.
I'm basing this on the idea that each candidate wants my vote, and by giving it to the one I support (in this case a third party), I've made the total for both primaries one vote smaller.
If you vote for one of the leading candidates, sure, your vote has delta 3 (but the -1 to the trailing candidate is basically irrelevant, because he can't win anyway).
If you vote for the trailing candidate, then from the perspective of the leading candidates, you have effectively zeroed your vote (you haven't given +1 or -1 to either candidate who can potentially be elected).